Our progress in the last years is in the line of research that we have been developing in the field of protein-tyrosine kinases. We have recently identified the major tyrosine phosphorylated protein observed after engagement of Fc-gamma receptors as p120c-cbl. We had already characterized this protein as an SH3 binding protein, because it specifically associates to the SH3 domains of p47nck. In the myelomonocytic system we have observed that p120c-cbl associates with the SH3 domain of the non-receptor protein-tyrosine kinase p56lyn. In another line of work, we have characterized cbl-b, a new proto- oncogene belonging to the c-cbl family, as an SH3 binding protein. Regulation of the phosphorylation of this protein by SH domains interactions is in progress. Protein-protein interactions have been analyzed in the tyrosine kinases p59fyn and p55fgr by mutational analysis. We have observed that these closely related kinases are very differently affected by mutations in very conserved residues of their SH2 and SH3 domains, suggesting that these domains regulate through various mechanisms the activity of these enzymes. We have also identified and characterized a second protein found to specifically associate to p47nck. This gene has been recently cloned by Derry et al. (Cell 78:635-644) and denominated WASP, because it has been found to be mutated in WAS (Wiskott-Aldrich Syndrome) patients. We have characterized the protein expressed by this gene as an SH3 binding protein of 66 kDa, which is present in the nucleus, cytosol and membrane of myelomonocytic cells. Recent work has shown that WASP binds the small GTP-binding proteins Rac1 and Cdc42, and that WASP might connect these GTPases to the actin cytoskeleton. Using the yeast-based two-hybrid system, we have recently identified a number of WASP-binding proteins, and work is in progress to characterize their function in biologically relevant settings.